In thermoplastic automotive air bag cover skins having an air bag deployment tear seam that is torn by the force of an inflating air bag to form an opening in the skin for deployment of the air bag, it is important that the skin not fragment along the edge of the opening created therein as such fragments could then enter the passenger compartment. Polyvinyl chloride (PVC) is for a number of well known reasons commonly used to form an automotive instrument panel shell or skin that includes a cover portion covering a passenger side air bag. However, PVC normally becomes brittle at about -20 .degree. C. and an air bag cover skin formed of this material and having such a tear seam is prone to fragment at the tear seam when the seam is torn for air bag deployment at temperatures falling below this level.
To avoid a possible fragmentation problem associated with a tear seam in a PVC air bag cover skin at extremely low temperatures, an entirely separate air bag deployment door may be added to the air bag cover. However, this can substantially add to the cost as compared to a relatively simple tear seam formed in the air bag cover skin and then there are the fit and finish considerations associated with this approach that can further increase the cost. Moreover, such a separately added air bag door is more prone to being damaged accidently or tampered with intentionally as compared to an integral air bag door defined by a tear seam in the cover skin. Furthermore, a separately added door is not suited to hiding its presence and thereby the presence of an air bag system whereas a tear seam in the cover skin can be hidden from view where such is desired by the automobile manufacturer.
The effect of embrittlement of PVC at extremely low temperatures when used to form an air bag cover skin having an air bag deployment tear seam can be minimized by a formed in place tear seam strip of lesser strength PVC or polyolefin material as disclosed in U.S. Pat. No. 5,288,103. In the manufacture thereof, a narrow tear seam gap defining gasket is pressed against the mold surface of a shell tool that defines the outer side of the skin. The skin is then formed in a casting process wherein the shell tool is heated and PVC powder including a suitable plasticizer and selected colorant is then cast from a powder box secured to the shell tool and onto the heated mold surface and about the gasket by inverting the shell tool and powder box. Upon formation of the skin to a controlled thickness on the mold surface, the shell tool and powder box are returned together to their former position wherein the then uprighted powder box retains the unused powder. The gasket is then removed with the powder box from the shell tool, the shell tool alone is rotated so that its mold surface faces upwardly, and the remaining gap left in the skin by the gasket is then filled by spraying the gap with a lesser strength thermoplastic material such as a highly glass filled or carbonate or other filled thermoplastic PVC or polyolefin material while the skin remains on the heated mold surface. The lesser strength PVC or polyolefin material is provided with the same color as the skin and on solidifying and curing on the mold surface forms a narrow tear seam strip in the skin that is substantially more frangible than the surrounding skin and wherein the skin including the tear seam strip may be covered with a coating of paint where the color match is not sufficient to make the tear seam invisible. The formed in place tear seam strip limits the potential fracture zone in the skin covering the air bag as compared to a tear seam formed by a groove in the skin as the skin in the latter case is more prone to fracture into segments or particles along and outward of the tear seam groove because of cold embrittlement.
In arriving at the method and apparatus of the present invention, it was recognized that while a formed in place PVC or polyolefin tear seam strip can provide satisfactory performance (i.e. no significant fragmentation) down to temperatures as low as about -20.degree. C., the use of PVC and polyolefin material limits low temperature tear seam performance capability as compared to thermoplastic urethane (TPU) that could extend such performance down to a much lower temperature of about -40.degree. C. However, thermoplastic urethane in a highly weakened form produced by a filler (referred to herein as a "filled TPU") is not well suited from a conventional processing standpoint to forming a tear seam strip in place in a powder cast PVC skin like in the above U.S. Pat. No. 5,288,103 or in a powder cast TPU skin because the filled TPU does not melt when using standard sprayed tear seam techniques. It was discovered that the combination of a suitably filled TPU tear seam strip insert with a PVC skin but more preferably with a TPU skin has the potential for significantly reducing the possibility of any fragmentation of the tear seam strip and the surrounding skin at extremely cold temperatures where a PVC skin with a formed in place PVC or polyolefin tear seam strip can not provide satisfactory performance. The tear seam may have various configurations such as a H, U or X-shape and the problem then is how to efficiently and cost effectively include in the casting of the skin, such a filled TPU tear seam strip insert of various configurations, or for that matter a filled PVC or polyolefin tear seam strip insert of various configurations where the low temperature operating requirements are less extreme. Moreover, it was found that this problem is compounded by the discovery that air can become entrapped between the heated mold surface and such an insert and result in highly undesirable cavities or pits in the outer surface of the insert.